In the measurement of caliper of paper, especially relatively thin paper, the use of air bearings has been thought to be unsatisfactory because of inherently poor accuracy. For example, because of the tendency of air bearings to compress due to forces and the passline of the moving sheet material, they are too inaccurate. For example, errors of 2-4% are common.
To reduce the error to the required less than 1%, prior techniques have tended to use some contacting type of caliper sensor. For example, one known type provides a measuring gauge on both sides of the paper with one side having a suction applied to it to maintain the paper flat. Again, this is a large area of contact and unless carefully done, tends to easily rip the paper. Another technique is the use of wheels which make contact with the paper. Here there are acceleration problems when the speed of the paper changes. Then another construction uses a pair of skids. All of the foregoing techniques have a type of contact with the moving paper which, when it has a relatively thin thickness or caliper, is very susceptible to marking or damage.